Precedence Networks - Construction Project Scheduling and Control - page 102

Figure 5.16 Three-activity project, with 9-day duration, using a combination relationship
Figure 5.17 Three-activity project, with 7-day duration, using a combination relationship
the activities, the FF relationships control. This control can change with a change in
duration, as shown in Figures 5.16 and 5.17.
Note that, in all the preceding cases—Figures 5.11 through 5.17, the duration
of any activity may be longer than the difference between its finish and start dates:
EF − ES or LF − LS . This means it may stretch across a period that is longer than its
duration (i.e., started, stopped, and then resumed), which is in accordance with our
assumption of interruptible activities.
The Simplistic Approach
CPM calculations for precedence diagrams are similar to, but not the same as, the
CPM calculations discussed in Chapter 4. The differences can be summarized by the
following three points:
1. In Chapter 4 (arrow and node diagrams), we had only the FS type of relation-
ship. Activities were always connected in only one way: from the end of an
activity's predecessor to its start, and from its end to the start of its successor.
In precedence diagrams, we use four types of relationships. Thus, you must
be careful about the type of relationship chosen. An activity can be controlled
(driven) not only from its start but from its end as well.
2. In precedence diagrams, we may use combination relationships (mainly SS and
FF). Doing so creates a condition in which each relationship yields a different
set of dates, but one set is rejected and the other prevails. Let us go through
the calculations for Figure 5.17, for example:
a. In the forward pass, activity A starts the project. ES = 0; EF = 0 + 5 (dura-
tion) = 5.